Ch 17 Gene Expression I: Transcription
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Transcript of Ch 17 Gene Expression I: Transcription
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Ch 17 Gene Expression I: Transcription
What is this?
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Basic Principles of Gene Expression
DNA encodes hereditary information (genotype) ->
decoded into RNA -> protein (phenotype)
DNA
RNA
Protein
Transcription
Translation
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LE 17-3-1
TRANSCRIPTIONDNA
Prokaryotic cell
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LE 17-3-1
TRANSCRIPTIONDNA
Prokaryotic cell
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LE 17-3-2
TRANSCRIPTIONDNA
Prokaryotic cellRibosome
Polypeptide
mRNA
Prokaryotic cell
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LE 17-3-3
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Prokaryotic cell
Nuclearenvelope
TRANSCRIPTION
Eukaryotic cell
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LE 17-3-4
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSCRIPTION
RNA PROCESSING
Eukaryotic cell
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LE 17-3-5
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Ribosome
Polypeptide
Eukaryotic cell
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• Promoter: DNA sequence where RNA polymerase binds to transcribe the gene
• Transcription start site: the nucleotide where RNA pol initiates transcription
• Transcription unit: the transcribed DNA
Transcription: DNA->RNA
Structure of a gene
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Basic components for transcription
dsDNA with a promoter
RNA polymerase
rNTPs (ribonucleotides triphosphates)ATP, CTP, GTP, UTP
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LE 17-7
Promoter Transcription unit
RNA polymeraseStart point
DNA
53
35
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LE 17-7
ElongationNon-templatestrand of DNA
RNApolymerase
RNA nucleotides
3 end3
5
5
Newly madeRNA
Templatestrand of DNA
Direction of transcription(“downstream”)
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Synthesis of an RNA Transcript
• The three stages of transcription:– Initiation– Elongation– Termination
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LE 17-7Promoter
35
Transcription unit
DNA
InitiationRNA polymerase
Start point
Template strandof DNA
RNAtran-script
UnwoundDNA
Elongation
3
3
53
5
5
3 5
RewoundDNA
5 3
35 35
RNAtranscript Termination
35
5 3Completed RNA transcript
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Termination of Transcription
Different in prokaryotes and eukaryotes• In prokaryotes• RNA pol stops transcription at the end of the
terminator (DNA sequence)
• In eukaryotes• pre-mRNA is cleaved from the growing RNA chain• RNA pol eventually falls off the DNA
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RNA processing in eukaryotes, not prokaryotes
1. Addition of methylated cap to 5’ end of messenger RNA (mRNA)-> increases stability and translation of mRNA
2. Addition of poly(A) tail to 3’ end (polyadenylation) -> increases stability and translation of mRNA
3. Splicingremoval of introns and joining together of exons
All processing events occur in nucleus
before transport to cytoplasm
Draw
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LE 17-10
5 Exon Intron Exon Intron Exon 3Pre-mRNA
1 30 31 104 105 146
Codingsegment
Introns cut out andexons spliced together
1 146
5Cap
5Cap
Poly-A tail
Poly-A tail
5 3UTR UTR
(mature) mRNA
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• RNA splicing:
carried out by spliceosomes
• Spliceosomes complex of proteins and several small nuclear
ribonucleoproteins (snRNPs)
Recognize splice sites (specific RNA sequences)
cleave out introns and splice together exons (coding region)
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LE 17-11
Exon 15
Intron Exon 2
Other proteinsProtein
snRNA
snRNPs
RNA transcript (pre-mRNA)
Spliceosome
5
Spliceosomecomponents
Cut-outintron
mRNA
Exon 1 Exon 25
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Ribozymes
• Catalytic RNAs molecules that function as enzymes; involved in splicing
• Non-protein biological catalyst
Can you think of a ribozyme with a different function?
Telomerase
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Functional and Evolutionary Importance of Introns
• Some genes can encode more than one kind of polypeptide
-different combinations of exons can be spliced together
• Called alternative RNA splicing
• Increases the potential number of different proteins (and thus functions) in an organism
• Increased adaptive potentialDraw Splice Variants
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• In many cases, different exons code for the different domains in a protein
• Protein domains– Distinct conformational regions often with discrete
functions
Exons and protein domains
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LE 17-12
Gene
Transcription
RNA processing
Translation
Domain 2
Domain 3
Domain 1
Polypeptide
Exon 1 Intron Exon 2 Intron Exon 3
DNA
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LE 17-9
5Protein-coding segment
5 Start codon Stop codon Poly-A tail
Polyadenylation signal
5 3Cap UTR UTR
Architecture of eukaryotic mRNA
UTR: untranslated regions